Carbohydrate Metabolism Practice Questions

Q: Glucose-6-phosphate dehydrogenase (G6PD) is primarily found in which cellular compartment?

Cytosol. **Explanation:** **1. Why Cytosol is Correct:** Glucose-6-phosphate dehydrogenase (G6PD) is the **rate-limiting enzyme** of the **Hexose Monophosphate (HMP) Shunt** (also known as the Pentose Phosphate Pathway). The HMP shunt is a unique pathway because it does not involve the mitochondria and occurs entirely within the **cytosol** of the cell. This location is strategic, as the pathway generates **NADPH**, which is required for cytosolic processes such as fatty acid synthesis, steroid synthesis, and the reduction of glutathione to protect cells against oxidative stress. **2. Why Other Options are Incorrect:** * **Mitochondria:** This compartment houses the TCA cycle, Beta-oxidation of fatty acids, and the Electron Transport Chain (ETC). G6PD is not involved in these oxidative phosphorylation processes. * **Golgi Apparatus:** This organelle is primarily involved in the post-translational modification, sorting, and packaging of proteins; it does not host the enzymes of primary carbohydrate metabolism. * **Endoplasmic Reticulum (ER):** While the ER is involved in protein synthesis (Rough ER) and lipid/steroid synthesis (Smooth ER), the initial enzymatic steps of the HMP shunt occur in the surrounding cytosol. (Note: Glucose-6-phosphatase is located in the ER, but G6PD is cytosolic). **3. Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzymopathy worldwide. It leads to **hemolytic anemia** triggered by oxidative stress (e.g., Fava beans, Primaquine, Infection) because RBCs lack mitochondria and rely solely on the HMP shunt for NADPH to maintain reduced glutathione. * **Tissue Distribution:** G6PD activity is highest in tissues requiring NADPH for reductive biosynthesis (Liver, Adipose tissue, Lactating mammary glands, Adrenal cortex) and in RBCs for antioxidant defense. * **Key Product:** The HMP shunt produces **Ribose-5-phosphate** (for nucleotide synthesis) and **NADPH** (not NADH).

Q: The oxidative phase of the HMP shunt pathway is least active in which of the following tissues?

Skeletal muscle. **Explanation:** The oxidative phase of the **Hexose Monophosphate (HMP) Shunt** (Pentose Phosphate Pathway) is primarily responsible for the production of **NADPH**. Therefore, this pathway is most active in tissues that require high amounts of NADPH for reductive biosynthesis or protection against oxidative stress. **Why Skeletal Muscle is the Correct Answer:** Skeletal muscle lacks the enzymes for significant fatty acid or steroid synthesis. Its primary metabolic requirement is ATP production via glycolysis and the TCA cycle. Since it does not perform large-scale reductive biosynthesis, the activity of Glucose-6-Phosphate Dehydrogenase (G6PD)—the rate-limiting enzyme of the oxidative phase—is extremely low in resting skeletal muscle. **Analysis of Incorrect Options:** * **Adrenal Cortex:** Highly active in the HMP shunt because NADPH is essential for the hydroxylation reactions involved in **steroid hormone synthesis**. * **Lactating Mammary Gland:** Requires massive amounts of NADPH for the **de novo synthesis of fatty acids** found in breast milk. * **Red Blood Cells (RBCs):** Utilize NADPH to maintain a pool of **reduced glutathione**, which is critical for neutralizing reactive oxygen species (ROS) and preventing hemolysis. **High-Yield Facts for NEET-PG:** * **Rate-limiting enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD), regulated by NADP+ levels. * **Key Products:** NADPH (for biosynthesis/antioxidant defense) and Ribose-5-phosphate (for nucleotide synthesis). * **Tissues with High HMP Activity:** Liver, Adrenal cortex, Testes/Ovaries, Lactating mammary gland, RBCs, and Phagocytic cells (for respiratory burst). * **Clinical Correlation:** G6PD deficiency leads to hemolytic anemia under oxidative stress (e.g., fava beans, primaquine) because RBCs cannot generate enough NADPH to maintain glutathione in its reduced state.

Q: NADPH is generated by the action of which enzyme?

Glucose 6 phosphate dehydrogenase. **Explanation:** The correct answer is **Glucose 6-phosphate dehydrogenase (G6PD)**. **1. Why G6PD is correct:** G6PD is the rate-limiting and regulatory enzyme of the **Hexose Monophosphate (HMP) Shunt** (also known as the Pentose Phosphate Pathway). This pathway occurs in the cytosol and is the primary source of **NADPH** in the body. G6PD catalyzes the oxidation of Glucose 6-phosphate to 6-phosphogluconolactone, simultaneously reducing $NADP^+$ to $NADPH$. This $NADPH$ is essential for reductive biosynthesis (e.g., fatty acids, steroids) and for maintaining the pool of reduced glutathione to protect cells against oxidative stress. **2. Why other options are incorrect:** * **Glucose 1-phosphate dehydrogenase:** This enzyme does not exist in human carbohydrate metabolism. Glucose 1-phosphate is an intermediate in glycogenesis and glycogenolysis, managed by enzymes like phosphoglucomutase. * **Glucose 1,6-diphosphate dehydrogenase:** This is not a recognized enzyme in the HMP shunt or major metabolic pathways. Glucose 1,6-bisphosphate acts primarily as a cofactor for the enzyme phosphoglucomutase. **Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzymopathy worldwide. Deficiency leads to decreased NADPH, causing oxidative damage to RBCs, resulting in **Heinz bodies**, **Bite cells**, and acute hemolytic anemia (often triggered by Fava beans, infections, or drugs like Primaquine). * **Tissues involved:** The HMP shunt is highly active in the liver, lactating mammary glands, adrenal cortex (for steroid synthesis), and RBCs (for antioxidant defense). * **Transketolase:** Another key HMP shunt enzyme; it requires **Thiamine (B1)** as a cofactor. Measuring its activity is used to diagnose Thiamine deficiency.

Q: A 30-year-old patient presents with intractable vomiting and inability to eat or drink for the past 3 days. Their blood glucose level remains normal. Which of the following is most important for the maintenance of blood glucose in this patient?

Liver. **Explanation:** The maintenance of blood glucose during fasting or starvation is primarily the responsibility of the **Liver**. In this patient, who has been unable to eat for 3 days, the body has transitioned from the post-absorptive state to the early starvation phase. **Why the Liver is Correct:** The liver is the central organ for glucose homeostasis. It maintains blood glucose through two key pathways: 1. **Glycogenolysis:** The breakdown of stored glycogen (exhausted within 12–24 hours of fasting). 2. **Gluconeogenesis:** The synthesis of glucose from non-carbohydrate precursors (lactate, glycerol, and glucogenic amino acids). By day 3 of starvation, gluconeogenesis in the liver is the **primary source** of blood glucose to support glucose-dependent tissues like the brain and RBCs. The liver uniquely possesses the enzyme **Glucose-6-Phosphatase**, allowing it to release free glucose into the bloodstream. **Why Other Options are Incorrect:** * **Heart:** The heart is a consumer of energy, not a producer. It primarily utilizes fatty acids and ketone bodies during starvation to spare glucose for the brain. * **Skeletal Muscle:** While muscle stores significant glycogen, it lacks **Glucose-6-Phosphatase**. Therefore, muscle glycogen can only be used locally for energy and cannot be released as glucose into the blood. * **Lysosome:** These are organelles involved in cellular degradation (autophagy) and do not play a direct role in systemic blood glucose regulation. **NEET-PG High-Yield Pearls:** * **Key Enzyme:** Glucose-6-Phosphatase is the "marker enzyme" for gluconeogenesis, found in the liver and kidney cortex, but absent in muscle. * **Timeline:** Liver glycogen is depleted in ~18 hours; thereafter, gluconeogenesis becomes the sole source of blood glucose. * **Kidney Role:** In prolonged starvation (>5–6 days), the kidney cortex also contributes significantly (up to 40%) to gluconeogenesis.

Q: All of the following are intermediates of the TCA cycle, except?

Malonate. **Explanation:** The **Tricarboxylic Acid (TCA) cycle**, also known as the Krebs cycle, occurs in the mitochondrial matrix and is the final common pathway for the oxidation of carbohydrates, lipids, and proteins. **Why Malonate is the Correct Answer:** **Malonate** is not an intermediate of the TCA cycle; rather, it is a potent **competitive inhibitor** of the enzyme **Succinate Dehydrogenase** (Complex II). It is a structural analogue of Succinate. By binding to the active site of the enzyme, malonate prevents the conversion of succinate to fumarate, effectively halting the cycle. This is a classic example of competitive inhibition frequently tested in biochemistry. **Analysis of Incorrect Options:** * **Alpha-ketoglutarate:** Formed from Isocitrate by *Isocitrate Dehydrogenase*. It is a key rate-limiting step and a precursor for glutamate synthesis. * **Succinate:** Formed from Succinyl-CoA by *Succinyl-CoA Synthetase* via substrate-level phosphorylation (producing GTP). * **Fumarate:** Formed from the oxidation of Succinate by *Succinate Dehydrogenase*. It is also a link to the Urea cycle and Tyrosine catabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Malonate vs. Malate:** Do not confuse the two. **Malate** is a TCA intermediate (formed from fumarate), while **Malonate** is the inhibitor. * **Succinate Dehydrogenase:** It is the only enzyme of the TCA cycle that is **integral to the inner mitochondrial membrane** (part of the Electron Transport Chain as Complex II). * **Fluoroacetate:** Another TCA inhibitor (found in some plants) that inhibits *Aconitase* after being converted to Fluorocitrate ("Suicide inhibition").

Q: Glucuronic acid and Iduronic acid are:

Epimers. ### Explanation **Correct Answer: D. Epimers** **Underlying Concept:** Epimers are stereoisomers that differ in configuration around only **one** specific carbon atom (other than the anomeric carbon). Glucuronic acid and Iduronic acid are both uronic acids derived from glucose. They are **C-5 epimers**. * In **D-Glucuronic acid**, the carboxyl group (-COOH) at C-5 is positioned above the ring (in the D-configuration). * In **L-Iduronic acid**, the configuration at C-5 is inverted. **Why other options are incorrect:** * **Anomers (A):** These differ only at the hemiacetal/hemiketal carbon (C-1 for glucose). Glucuronic and Iduronic acids differ at C-5. * **Enantiomers (B):** These are non-superimposable mirror images where every chiral center is inverted (e.g., D-Glucose and L-Glucose). These two acids are not mirror images. * **Functional Isomers (C):** These have the same molecular formula but different functional groups (e.g., Glucose and Fructose). Both molecules here are uronic acids. **Clinical Pearls for NEET-PG:** 1. **GAG Composition:** Iduronic acid is a major component of Glycosaminoglycans (GAGs) like **Heparin** and **Dermatan sulfate**. 2. **Epimerization:** In the body, D-glucuronic acid residues in heparin chains are converted to L-iduronic acid by the enzyme **uronyl C-5 epimerase**. 3. **Other High-Yield Epimers:** * **Glucose and Galactose:** C-4 epimers. * **Glucose and Mannose:** C-2 epimers. * **Ribulose and Xylulose:** C-3 epimers. 4. **Glucuronic Acid Function:** It is essential for the **conjugation of bilirubin** and the detoxification of xenobiotics in the liver.

Q: What is the amphibolic cycle?

Citric acid cycle. **Explanation:** The **Citric Acid Cycle (TCA cycle or Krebs cycle)** is termed an **amphibolic cycle** because it plays a dual role in metabolism, involving both **catabolic** (breakdown) and **anabolic** (synthetic) pathways. 1. **Catabolic Role:** It is the final common oxidative pathway for carbohydrates, fats, and amino acids, where Acetyl-CoA is oxidized to $CO_2$ and $H_2O$ to generate energy (ATP, NADH, $FADH_2$). 2. **Anabolic Role:** Intermediates of the cycle serve as precursors for various biosynthetic pathways. For example: * **Succinyl-CoA** is used for Heme synthesis. * **$\alpha$-Ketoglutarate** and **Oxaloacetate** are used for synthesis of non-essential amino acids (via transamination). * **Citrate** is exported to the cytosol for fatty acid and cholesterol synthesis. **Why other options are incorrect:** * **Glycolysis:** Primarily a **catabolic** pathway that breaks down glucose into pyruvate. While some intermediates can divert to other pathways, its primary physiological role is energy production. * **Protein Synthesis:** A purely **anabolic** process (building proteins from amino acids). * **Lipolysis:** A purely **catabolic** process (breakdown of lipids into fatty acids and glycerol). **High-Yield NEET-PG Pearls:** * **Anaplerotic Reactions:** Since TCA intermediates are consumed for biosynthesis, they must be replenished. The most important anaplerotic reaction is the conversion of **Pyruvate to Oxaloacetate** by *Pyruvate Carboxylase* (requires Biotin). * **Location:** The TCA cycle occurs entirely in the **mitochondrial matrix**. * **Key Regulatory Enzyme:** Isocitrate Dehydrogenase (rate-limiting step).

Q: McArdle's disease is due to deficiency of which enzyme?

Myophosphorylase. **Explanation:** McArdle’s disease, also known as **GSD Type V**, is an autosomal recessive disorder characterized by the deficiency of **Myophosphorylase** (muscle glycogen phosphorylase). This enzyme is responsible for the rate-limiting step of glycogenolysis in skeletal muscle, breaking down glycogen into glucose-1-phosphate. Without it, muscles cannot mobilize glucose during strenuous exercise, leading to exercise intolerance, muscle cramps, and myoglobinuria. **Analysis of Options:** * **A. Myophosphorylase (Correct):** This is the muscle-specific isoform of glycogen phosphorylase. Its absence prevents the breakdown of muscle glycogen. * **B. Liver phosphorylase:** Deficiency of this enzyme leads to **Hers disease (GSD Type VI)**, which primarily presents with hepatomegaly and mild hypoglycemia, rather than muscle symptoms. * **C. Glucose-6-phosphatase:** Deficiency leads to **Von Gierke’s disease (GSD Type I)**, the most common GSD, characterized by severe fasting hypoglycemia, lactic acidosis, and "doll-like" facies. * **D. Acid maltase (α-1,4-glucosidase):** Deficiency leads to **Pompe disease (GSD Type II)**, which affects the heart and muscles due to lysosomal glycogen accumulation. **High-Yield Clinical Pearls for NEET-PG:** 1. **Ischemic Forearm Exercise Test:** Patients with McArdle’s show a **failure of blood lactate to rise** after exercise (since glycogen cannot be converted to lactate), while ammonia levels rise significantly. 2. **Second Wind Phenomenon:** Patients often experience a sudden improvement in exercise tolerance after 10–15 minutes as the body switches to using free fatty acids and blood glucose for energy. 3. **Burgundy-colored urine:** Post-exercise myoglobinuria can lead to acute renal failure.

Question 1

Glucose-6-phosphate dehydrogenase (G6PD) is primarily found in which cellular compartment?

Accepted Answer

Cytosol

Answer

Mitochondria

Answer

Golgi apparatus

Answer

Endoplasmic reticulum

Question 2

The oxidative phase of the HMP shunt pathway is least active in which of the following tissues?

Accepted Answer

Skeletal muscle

Answer

Adrenal cortex

Answer

Lactating mammary gland

Answer

Red blood cells

Question 3

A 30-year-old male has excessive urinary glucose levels, but his blood glucose levels are normal. Investigations reveal high levels of L-xylulose in urine. A genetic defect in which of the following pathways is most likely responsible?

Accepted Answer

Uronic acid pathway

Answer

Tricarboxylic acid cycle

Answer

Glycolysis

Answer

Hexose monophosphate shunt

Question 4

What is the primary action of epinephrine on the liver?

Accepted Answer

Glycogenolysis

Answer

Glycolysis

Answer

Lipolysis

Answer

Gluconeogenesis

Question 5

NADPH is generated by the action of which enzyme?

Accepted Answer

Glucose 6 phosphate dehydrogenase

Answer

Glucose 1 phosphate dehydrogenase

Answer

Glucose 1,6 diphosphate dehydrogenase

Answer

All of the above

Question 6

A 30-year-old patient presents with intractable vomiting and inability to eat or drink for the past 3 days. Their blood glucose level remains normal. Which of the following is most important for the maintenance of blood glucose in this patient?

Accepted Answer

Liver

Answer

Heart

Answer

Skeletal muscle

Answer

Lysosome

Question 7

All of the following are intermediates of the TCA cycle, except?

Accepted Answer

Malonate

Answer

alpha-ketoglutarate

Answer

Succinate

Answer

Fumarate

Question 8

Glucuronic acid and Iduronic acid are:

Accepted Answer

Epimers

Answer

Anomers

Answer

Enantiomers

Answer

Functional isomers

Question 9

What is the amphibolic cycle?

Accepted Answer

Citric acid cycle

Answer

Glycolysis

Answer

Protein synthesis

Answer

Lipolysis

Question 10

McArdle's disease is due to deficiency of which enzyme?

Accepted Answer

Myophosphorylase

Answer

Liver phosphorylase

Answer

Glucose-6-phosphatase

Answer

Acid maltase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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